The present invention relates to a novel smectic liquid crystal mixture and a novel active matrix liquid crystal device or display. More particularly, it relates to chiral smectic or more specifically ferroelectric liquid crystal mixtures, which enables the generation of gray levels and full color representation useful for computer monitors, TV, DVD, Video and other displays. In particular these displays are useful for high speed applications and for yielding a strong colour saturation by improved backlight techniques which rely upon a high speed liquid crystal. A further aspect of this invention are active matrix displays containing such a mixture, particularly in a monostable geometry.
Monostable chiral smectic liquid crystals have been proposed to be combined with active matrix technology to simultaneously allow the utilization of a very high xe2x80x98pixel speedxe2x80x99 and show a gray scale, contrary to bistable SSFLC displays.
Takatoh et. al. (6th International Conference on Ferroelectric Liquid Crystals, Jul. 20-24, 1997, Brest, France; M. Takatoh et al. 1998, SID Digest, 1171-1174) have demonstrated an AM display based upon chiral smectics using a very high Ps material driven with an active matrix with polycrystalline Silicon-TFT. Nito et. al. (Nito et al., 1993, Journal of the SID, 1/2,163-169.) have suggested a monostable AM-FLC with much lower Ps, however, with the disadvantage of a stripey FLC texture which is not suitable for high contrast displays without further improvements. Furue et. al. (Furue, H. et al., 1998, IDW ""98, 209-212) suggested a polymer stabilized SSFLCD with a FELIX(copyright) mixture with a material having a moderate Ps value.
Asao et. al. have presented a monostable FLC mode (Y. Asao et al., ILCC 2000, Sendai, and Jpn. J. Appl. Phys. 38, L534-L536, 1999 therein called xe2x80x9chalf-V-shape FLCxe2x80x9d mode; see also T. Nonaka et. al., Liquid Crystals 26(11), 1599-1602, 1999, therein called xe2x80x9cCDRxe2x80x9d mode). Such displays provide, by virtue of their smaller Ps values, solutions for the gray scale problem and the resolution limitation caused by too large Ps values in active matrix panels.
A remaining problem in the application of TFT-FLCD (e.g. monostable FLC) is the limited so-called xe2x80x9cimage stickingxe2x80x9d effect (H. Pauwels et. al., Proceedings of the IDRC, Florida 2000, pp.72-75), caused by the activity of the manifold of charge carriers that are present in the pixel volume and which tend to generate a hysteresis effect, i.e. the type of gray level becomes dependent on the pixel switching history and the correct addressing of gray levels becomes therefore hardly reproducible in practice.
Thus it is seemingly an important factor to maintain a very low ion content and to achieve chemical stability against heat and light, both of which could cause additional ion formation. This demand has in practice lead to the exclusion of any material containing hetero atoms such as N, S, even O in liquid crystal mixtures for active matrix (i.e. TFT or MIM) applications {cp. e.g. Petrov et al., Liq. Cryst. 19(6), 729 (1995) [CAN 124:101494]; Petrov, Proc. SPIE-Int. Soc. Opt. Eng.(1995), 2408 [CAN 123:241500]; Dabrowski, Biul. Wojsk. Akad. Techn. 48(4), 5 (1999) [CAN 131:163227]; Kirsch, Angew. Chem., Int. Ed. 39(23),4216 (2000) and references cited in these papers}. Whereas, so far, this could successfully be done for nematics (cp. e.g. DE-A 1 962 9812, p. 12 to 16), there is hardly any smectic (Sc) material without such hetero atoms. Thus the use of fast switching smectics for TFT application is strongly limited if not prohibited if the condition to exclude heteroatoms would be mandatory.
The objective of the present invention was therefore to provide a smectic liquid crystal mixture and an active matrix liquid crystal display with very high reproducibility of the gray levels irrespective of the containment of hetero atoms, in particular ferroelectric or antiferroelectric liquid crystal mixtures.
Contrary to the above-mentioned approach to keep liquid crystal materials free of ionic decontaminations, we have surprisingly found that certain dopants as additives to smectic liquid crystal mixtures are indeed effective in removing the image sticking in active matrix displays, while keeping a sufficiently high holding ratio. This is insofar unexpected, as the dopants, e.g. polyethylene glycoles, crown ethers, aza-crown ethers and the like usually contain several oxygen and/or nitrogen atoms and increase the conductivity of the material. Thus, in the traditional view, these dopants should cause a strong image sticking and a low holding ratio. Surprisingly, the opposite is observed.
Thus, the present invention provides a smectic liquid crystal mixture comprising a smectic liquid crystal base mixture and at least one dopant, which is either a complex ligand or an organic material in a concentration of 0.01 to 5.0 weight percent. The electrical conductivity of the instant liquid crystal mixtures is increased by at least 20 percent with respect to the base mixture
In a preferred embodiment of the instant invention the dopant is present in a concentration of 0.05 to 1.0 weight percent, which leads to a suppression or minimization of the image sticking effect up to 10% of the untreated value. The liquid crystal mixture according to the invention can be used in active liquid crystal displays.
In particular the present invention relates to a ferroelectric or antiferroelectric liquid crystal mixture, most particularly a monostable ferroelectric liquid crystal mixture, especially a monostable ferroelectric liquid crystal mixture having a phase sequence with decreasing temperature
isotropic-tilted smectic
or
isotropic-nematic or cholesteric-tilted smectic
or
isotropic-nematic or cholesteric-smectic A-tilted smectic,
comprising one or several dopants.
For dopants according to invention with xe2x80x94OH or xe2x80x94NH2xe2x80x94 or xe2x80x94NHxe2x80x94 or xe2x80x94SH groups, for instance those described in EP-B-385 688, U.S. Pat. No. 5,122,296 and U.S. Pat. No. 6,139,925, which are hereby incorporated by reference.
The complex ligands according to the invention are either podands, coronands or cryptands. Pertinent terminology and listing of pertinent compounds is hereby incorporated by reference to EP-B-385 688, EP-B-502 964 and EP-B-599 928.
The complex ligands can also be xcex1-cyclodextrin [10016-20-3], xcex2-cyclodextrin [7585-39-9] or xcex3-cyclodextrin [17465-86-0].
A preferred group of podands encompasses oligoethylendialkylether and polyethylenglycols, respectively, each preferably with a molar mass above 350 amu, both sub-groups optionally substituted by pendant groups.
A preferred group of coronands encompasses crown ethers (i.e. medio- and macro-cycles with oxygen donors), aza crown (i.e. medio- and macro-cycles with nitrogen and oxygen donors), each sub-group optionally substituted by pendant groups.
Especially preferred are coronands with one or two nitrogen donors, these nitrogen atoms being substituted by xe2x80x94C(xe2x95x90O)xe2x80x94R groups wherein R is an alkyl group (linear or branched) or a cyclic moiety; with this respect reference is made to EP-B-599 928 which is hereby incorporated by reference.
A prefered group of cryptands encompasses bicyclic species with nitrogen and oxygen donors (e.g. kryptofix(copyright)), bicyclic species exclusively with oxygen donors, and tricyclic species (e.g. soccerball cryptand) with either nitrogen and oxygen donors or exclusively oxygen donors; all sub-groups can be substituted by pendant groups.
In either group of preferred complex ligands their properties can be adjusted to the additional parameters of the display by modifying the pattern of the podand, coronand and cryptand, resp., through incorporation of hetero atoms (other than nitrogen and oxygen, e.g. sulfur or silicium) as donors, through incorporation of cyclic donor moieties (e.g. pyridine or quinoline end groups in case of podands; e.g. phenylen-diyl in case of benzo crowns) and through attaching pendant groups, respectively. Combinations of these different types of modifications are also possible.
In particular the sum of the concentrations (in weight percent=wt %) of the dopant is below 10 wt %, preferably below 5 wt %, especially below 3 wt %. Preferably, the average molar mass of the dopants (or in case of one dopant the molar mass) is between 100 and 2000 amu, especially between 200 and 1000 amu, in particular between 200 and 600 amu.
For aza coronands, e.g. as disclosed in EP-B 599928, a preferred range is between 300 and 700 amu.
A further object of the invention is to provide a smectic liquid crystal display device, operated in an active matrix panel comprising the above described mixtures.
A further object of the invention is to provide a ferroelectric liquid crystal display device operated in an active matrix panel using the above described mixtures. With preference this display is a monostable ferroelectric display, such as half-V-shape, CDR or short pitch FLC displays.
A further object of the invention is to provide an antiferroelectric liquid crystal display device operated in an active matrix panel using the above described mixtures. With preference this display is a monostable antiferroelectric display, such as the so-called xe2x80x9cV-shapexe2x80x9d mode.
A further object of the invention is the use of the above described smectic mixtures operated in an active matrix panel, especially if the liquid crystal is a chiral smectic, particularly a monostable chiral smectic mode.
The liquid crystal mixtures according to the invention are prepared in a manner which is customary per se. As a rule the components are dissolved in one another, advantageously at elevated temperatures.
The liquid crystal mixtures according to the invention generally comprises at least two (2), preferably at least five (5), particularly at least eight (8) compounds.
As an alternative to add the dopants into the liquid crystal mixture, they can be mixed into or added onto the alignment layer, so that the dopants are effectively in contact with the smectic liquid crystal.
Optional additional constituents of the mixtures according to invention are materials that increase the light stability [UV stabilizers, e.g. of the xe2x80x9cbenzophenonexe2x80x9d type, xe2x80x9cbenzotriazolexe2x80x9d type or HALS (hindered amine light stabilizer) type]. Preferably the mixtures may contain 0.01 wt.-% to 10 wt.-% of one or several UV stabilizers; especially preferred are mixtures containing 0.1 wt.-% to 5 wt.-% of one or several UV stabilizers.
Of the HALS type several species have been disclosed for a variety of applications, e.g. in U.S. Pat. No. 4,038,280 or Sedlar, Polym. Photochem. (1982), 2(3), 175 [CAN 97: 24546].
There are several kinds of commercially available HALS compounds (e.g. provided by Clariant and Ciba Specialty Chemicals), mainly used for modifying UV stability for plastics and paints. These compounds are not specially designed for smectic liquid cystal materials, so that their use may induce decrease of phase transition temperatures of the liquid crystal or cause miscibility problems, all depending on the structures of the liquid crystal compounds and HALS compounds, resp.
More specifically, HALS compounds for use in liquid crystal mixtures have been disclosed in WO 02/18515 which is hereby incorporated by reference.
A preferred group of HALS compounds encompasses the formulae (Ia) to (Id) and (II) 
wherein
n is an integer from 1 to 20
X is H
R1 is H or alkyl of 1 to 4 C atoms
R3, R4, R5, R6 are CH3
wherein
X is H
R1 is H or alkyl of 1 to 4 C atoms
R3, R4, R5, R6 are CH3
wherein
X is H
R1 is H or alkyl of 1 to 4 C atoms
R3, R4, R5, R6 are CH3
wherein
X is H
R1 is H or alkyl of 1 to 4 C atoms
R3, R4, R5, R6 are CH3
In either case R2 is H, F, a linear or branched alkyl group of 1 to 20 C atoms or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one or two xe2x80x94CH2xe2x80x94 groups can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Si(CH3)2xe2x80x94, it also being possible to replace one or more H of the alkyl or alkenyl group by F or CH3 or a radical 
R3, R4, R5, R6: represent independently from another an alkyl group of 1 to 8 C atoms
M1, M2 are independently from another for a single bond, xe2x80x94OC(xe2x95x90O), xe2x80x94C(xe2x95x90O)Oxe2x80x94,
xe2x80x94OCH2xe2x80x94, or xe2x80x94NHxe2x80x94
M3 is a single bond, xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CH2CH2xe2x80x94,
xe2x80x94CH2CH2CH2CH2
are independently from another phenylen-1,4-diyl, which optionally can be substitued by one, two or three F, or cyclohexan-1,4-diyl, which can optionally be substituted by one CN, CH3 or F, or pyrimidin-2,5-diyl, optionally substituted by one F, pyridine-2,5-diyl, which can optionally be substituted by one F, or naphthalene-2,6-diyl, which can optionally be substituted by one, two or three F, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionally substituted by one, two or three F), or decahydronaphthalene-2,6-diyl, or indane-2,5(6)-diyl, or fluorene-2,7-diyl, or phenanthrene-2,7-diyl, or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or (1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl, or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, or piperazin-1,4-diyl 
wherein
R1 is H or a linear or branched alkyl group of 1 to 20 C atoms or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one xe2x80x94CH2xe2x80x94 group can be replaced by cyclohexylen-1,4-diyl, or one or two xe2x80x94CH2xe2x80x94 groups can be replaced by xe2x80x94Oxe2x80x94, if non-adjacent to N, or by xe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94, and one or more H of the alkyl or alkenyl group optionally being replaced by F or CH3.
R2 is
a) H or F
b) a linear or branched alkyl group of 1 to 20 C atoms or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one or two xe2x80x94CH2xe2x80x94 groups can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Si(CH3)2xe2x80x94, it also being possible to replace one or more H of the alkyl or alkenyl group by F or CH3.
c) a radical 
wherein independently from the respective meanings in (I)
R3, R4, R5, R6: represent independently from another an alkyl group of 1
to 8 C atoms
M1, M2 are independently from another for a single bond, xe2x80x94OC(xe2x95x90O), xe2x80x94C(xe2x95x90O)Oxe2x80x94,
xe2x80x94OCH2xe2x80x94, or xe2x80x94NHxe2x80x94
A is the group xe2x80x94C(xe2x95x90Y)xe2x80x94 wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by 1 to 3 halogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M1 and M2 are xe2x80x94C(xe2x95x90O)Oxe2x80x94 and xe2x80x94OC(xe2x95x90O)xe2x80x94, respectively;
X is H, OH, or a linear or branched alkyl or alkyloxy group of 1 to 20 C atoms, wherein one or two xe2x80x94CH2xe2x80x94 groups can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Si(CH3)2xe2x80x94, it also being possible to replace one or more H by F or CH3 
m: is 0 or 1
By matching of LC composition, type of complex ligand and type of light stabilizer mixtures can be achieved that withstand light-induced stress while the mixture and display performance (e.g. minimal hysteresis in monostable FLCDs, high contrast, stable high-quality alignment, high voltage-holding-ratio, addressability) remain in a technically required range.
With this respect favorable combinations can be derived from
LC mixtures e.g. as disclosed in WO99/64538, WO00/36054 or DE 19934799
and
coronands as disclosed in EP 385688, EP 502964 and EP 599928
and
2,2,6,6-tetramethyl-piperidine-4-yl compounds as disclosed in WO 02/18515
Preferred compositions comprise
LC mixtures e.g. as disclosed in WO99/64538, WO00/36054 or DE 1993479
and
aza coronands as disclosed in EP 599928
and
2,2,6,6-tetramethyl-piperidine-4-yl compounds as disclosed in WO 02/18515
Optional additional constituents of the mixtures according to invention are materials that increase the stability against oxidative degradation (antioxidants, e.g. of the xe2x80x9csterically hindered phenolxe2x80x9d type). Preferably the mixtures may contain 0.01 wt.-% to 10 wt.-% of one or several antioxidants; especially preferred are mixtures containing 0.1 wt.-% to 5 wt.-% of one or several antioxidants.
Optionally the mixtures according to invention may contain a combination of UV stabilizers and antioxidants.
Chiral smectic liquid crystal mixtures according to the invention are particularly suitable for use in electro-optical switching and display devices (displays). These displays are usually constructed in such a way that a liquid crystal layer is enclosed on both sides by layers which are usually, in this sequence starting from the LC layer, at least one alignment layer, electrodes and a limiting sheet (for example of glass). In addition, they can contain spacers, adhesive frames, polarizers and, for color displays, thin color-filter layers or are operated in the sequential backlight technique. Other possible components are antireflection, passivation, compensation and barrier layers and, for active-matrix displays, electric non-linear elements, such as thin-film transistors (TFTs) and metal-insulator-metal (MIM) elements. The structure of liquid crystal displays has already been described in detail in relevant monographs (see, for example, T. Tsukuda, xe2x80x9cTFT/LCD Liquid crystal displays addressed by thin film transistorsxe2x80x9d, Japanese Technology Reviews, 1996 Gordon and Breach, ISBN 2-919875-01-91.
An active matrix liquid crystal display within the scope of this invention can also be an optical element that acts as a light valve or a device used to change the polarization state of light by means of a field-induced re-orientation of the smectic liquid crystal molecules, in particular an FLCOS (ferroelectric liquid crystal on silicon), an optical shutter and the like.
In a preferred embodiment the FLC display is operated in the monostable mode with active matrix panel.
Several documents are cited in this application, e.g. to discuss the state of the art, synthesis of compounds used in the present invention or application of the mixtures according to the invention. All these documents are hereby incorporated by reference.